I have two tank heaters -- 800w and 1000w -- I'm using plugged into a heater controller I now know has a 10a fuse. The model says its rated for 800w, but I tested it before fish were in and it worked a treat with both (so I thought).There was also a ~100w fountain pump on the same circuit.

This was all running through a 16 gauge 100' extension cord. I noticed something was wrong when I touched the plug where it goes into the wall and it was hot. I immediately (the next day) run out and get a 12 gauge 50' extension cord.

It runs for about 10 minutes and blows my 15a fuse in my home fuse box. The fuse is ancient. True Value Hardware carries them so I get two. I move the pump to its own circuit (both heaters are running off the same controller).

It runs for about 10 minutes and blows the 10a fuse in the heater controller (as it should).

HOW DID THIS EVER WORK? Was all that energy just getting wasted as heat and it never transferred as much power as my setup needed?

Can I run all this on one 15a circuit at all? Am I going to have to separate the two heaters?

What voltage are you using? Power is voltage times current, so if you need to draw 1800 watts at 120 volts, you need 15 amps of current. Adding another 100 watts for the pump pushes you up to nearly 16 amps. If your voltage is below 120 volts, you need more current. If you are working at 220 volts, you only need about 8.6 amps total.

16 gauge wire has more resistance than 12 gauge wire, and 100 feet has more resistance than 50 feet. Resistance lowers the current draw (and wastes energy by causing unuseful heat). If you are on the edge of tripping a breaker, that resistance could be keeping you under the tripping current. However it is also a fire hazard due to the heat in the wires and connections.

So, it looks like you are just overloading the circuit, and should not do this. You need to split the load or get a heftier circuit (with heftier house wiring!)

Jose

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In case you are wondering what happened, the power rating on the devices are designed to draw 800W or 1000W from 120VAC (assuming US power sources, I don't know if True Value stores are common outside the US). The catch is that extension cord knocked the voltage down a bit, so they weren't heating nearly as much as 800W and 1000W. Assuming 100 feet of extension cable was warm to the touch, they probably weren't even close to that.

Counterintuitively, power draw decreases as resistance decreases: the Power=V**2/R. So once you replaced your high resistance extension cord with a low resistance one, you power draw went up along with the current (and the fuse blew). So as already mentioned in this thread, you need to spread out your power draw to separately fused lines.